1. Technical Field
The invention relates generally to wireless communication systems and, more particularly, to methods for encoding and decoding information that is transmitted in control channels in such systems.
2. Description of Related Art.
In wireless communication systems, an air interface is used for the exchange of information between a user equipment (UE) and a base station or other communication system equipment. The air interface typically comprises a plurality of communication channels. In the well-known High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard, for example, a High Speed Downlink Shared Channel (HS-DSCH) is used for transmissions from a base station to a plurality of UEs. To facilitate data transmission via a HS-DSCH, signaling information is provided via shared control channels. High Speed Shared Control Channels (HS-SCCHs) are associated with the HS-DSCHs.
HS-SCCHs are used for transmitting signaling information that is needed for the UE to process the corresponding data transmission. By way of example, signaling information in the HS-SCCHs can include transmission format information such as code information (which codes are used for the data transmission), modulation information, Transport Block Size (TBS), and so on. The HS-SCCHs are used on a shared basis among all the UEs such that all the UEs would read all the HS-SCCHs configured in a cell of a wireless network.
In the evolving wireless data communication systems, such as the well-known 1x-EV-DO and 1xEV-DV standards and the aforementioned High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard, a scheduling function is moved from a base station controller to the UEs in order to provide “fast” scheduling based on channel quality feedback from the UEs. Moreover, technologies such as adaptive modulation and coding (AMC) and hybrid automated repeat request (HARQ) have been introduced to improve overall system capacity. In general, a scheduler selects a UE for transmission at a given time and adaptive modulation and coding allows selection of the appropriate transport format (modulation and coding) for the current channel conditions seen by the UE.
In HSDPA, for example, the scheduler, AMC and HARQ functions are provided by a MAC-hs (medium access control—high speed) controller located in a base station. The MAC-hs is responsible for handling the data transmitted on the air interface. Furthermore the MAC-hs has responsibility to manage the radio link physical resources allocated to HSDPA. In general, the functions carried out by MAC-hs include flow control, scheduling/priority handling, Hybrid ARQ, and a physical layer transport format, e.g., modulation, coding scheme, etc.
In order to enable the above-mentioned technologies, control signaling is needed both on the uplink (UE to base station) and downlink (base station to UE). The uplink signaling consists of ACK/NACK feedback for HARQ operation and channel quality indication (CQI). The uplink signaling in HSDPA is carried over a high speed dedicated physical control channel (HS-DPCCH). In the downlink signaling for HSDPA, the HS-SCCH is used to carry the scheduling and HARQ control information for the current transmission to the UE.
Control or signaling information in the signaling message that is transmitted via a HS-SCCH is typically encoded, e.g., with block codes or convolutional codes. As such, a UE must decode all the information in the HS-SCCH in order to decode the signaling message, which is then used for processing the corresponding data transmission over a corresponding HS-DSCH.
FIG. 1 illustrates the relationship between HS-SCCHs 110 and their corresponding shared HS-DSCH counterparts 120. In FIG. 1, each HS-SCCHx (x=1 to 4) carries signaling message information pertinent to a corresponding HS-DSCHx (x=1 to 4). The number of HS-DSCHs, and therefore the number of HS-SCCHs that may be used, can vary for each transmission time interval (TTI), depending on the number of UEs being simultaneously scheduled in the TTI. Accordingly, the configuration of HS-SCCHs and HS-DSCHs in FIG. 1 enables data channelization signaling codes and power resources to be divided among four simultaneous transmissions.
Referring again to FIG. 1, control channel data on each HS-SCCH is typically divided into two parts. Part I, as will be explained further, consists of control or signaling information related to data channelization signaling codes that have been assigned to a particular UE, for example. Part II, as will be explained further, contains HARQ related information, and other transport information.
The control signaling described above currently suffers from several disadvantages, namely higher error rates, miss/false alarm probabilities and inefficient resource usage. These problems are due to the separate encoding that is required for each of the shared control channels. With separate coding, each shared control channel carries, for example, cyclic redundancy code (CRC) data bits and tail bits separately for each UE in a cell of the network that is to receive a data transmission from a base station, for example.
FIG. 2 illustrates the contents of Part 1 and Part 2 in a signaling message that is to be transmitted over each HS-SCCH in further detail Part 1 and Part 2 contain several segments. For each HS-SCCH, Part 1 contains an information bit segment having a unique set of information bits (Info1HS-SCCHx, where x=1 to 4, a cyclic redundancy check code (CRC) segment that is used for error detection as is known, and a tail bits segment that terminate Part 1 of the HSSCCH. For example, the information bits may include a 7-bit channelization code signaling, a 1-bit modulation code, 10-bit UE-ID code of a single UE-ID, and other control or signal bit information. Similarly Part 2 of each HS-SCCH has a unique set of information bits (Info2 HS-SCCHx, where x=1 to 4, cyclic redundancy check code (CRC) bits and tail bits. The information bits in Part 2 may include HARQ-related data, transport format and resource related data such as TBS and other control information, as well as UE-ID and CRC information for a single UE.
To maintain complexity low at the UE, HS-SCCH designs typically allow Part I information to be transmitted prior to the commencement (i.e., before t=0) of data transmission, as shown in FIG. 1. With the current configuration, each UE must decode each Part I on each HS-SCCH, in every TTI, in order to determine (a) whether or not the transmission was intended for that particular UE, and (b) if the transmission was intended for that particular UE, the UE must decode Part I and figure out what channelization codes the corresponding HS-DSCH will arrive on. In other words, a UE must separately decode each Part 1 and Part 2 in order to fully decode the HS-SCCH that is intended for it, so that the UE may begin to buffer the intended transmission data over the HS-DSCH that corresponds to the successfully decoded HS-SCCH.
Accordingly, each UE must decode up to four (4) HS-SCCHs in every TTI, prior to commencement of data transmission. From a UE processing complexity perspective, it is therefore desirable to limit the number of bits in Part 1 that require processing, and also desirable that the processing be as simple as possible.
FIG. 3 illustrates a HSDPA transmission time interval (TTI) for a HS-SCCH. The TTI 300 comprises 3-timeslots 310a to 310c, each of a duration 0.667 ms each. Slot 310a contains Part 1 information, and slots 310b-c contain Part 2 information. FIG. 3 also illustrates the arrangement of channelization codes for transmitting the Part 1 and Part 2 information of a HS-SCCH. The HS-SCCH 350 information is transmitted over three channelization code slots, being divided into Part 1 (transmitted in the code in slot 360) and part 2 information (transmitted in the code in slots 370a-b). Accordingly, Part 1 information of 310a is transmitted within the first slot 360 and Part 2 information 310b-c are transmitted in the second and third channelization code slots 370a and 370b, as shown by the dotted arrows in FIG. 3. Each HS-SCCH uses a channelization code of spreading factor (SF) 128. With QPSK modulation and a chip rate of 3.84 Mc/s in UMTS, 40 bits are transmitted in a single timeslot.
The details of HS-SCCH control fields for HSPDA, i.e., the information bits and CRC bits of Parts 1 and 2, are summarized in Table 1. Note that a UE ID is not explicitly included in the control fields of Part 1 or Part 2, but an x-bit CRC is calculated over the control fields of Part 1 and 2, where x=8, 16, 24 or 32 CRC bits. In Table 1, the CRC code has a 16-bit length.
TABLE 1HS-SCCH InformationSCCH ControlSizeField[Bits]Transport-format andChannelization code set7Resource relatedModulation1Information (TFRI)Transport block set size and6transport channel identityHybrid-ARQ-relatedHybrid-ARQ process3Information (HARQnumberinformation)Redundancy version3New-data indicator1CRC16-bits
FIG. 4 illustrates an example of a UE-specific CRC calculation. One way of calculating a UE-specific CRC is to append the UE ID 410 with the other control fields 420 and perform a standard CRC calculation 430. At the time of transmission, the UE ID 410 is removed from the control fields 240 of Part 1 and Part 2, and the control fields 420 along with the calculated CRC 430 are transmitted (see line 435). When a UE receives a HS-SCCH transmission, it will perform the CRC check 440 by appending its own UE ID with the other control fields in Part 1 and separately in Part 2. If the CRC checks with the transmitted CRC, the UE assumes that the transmission is intended for the UE . If the CRC does not check with the transmitted CRC, the UE will ignore the transmission on the corresponding HS-DSCH.
Referring again to FIG. 2, The total number of bits in Part 1 for a single HS-SCCH is given by the expression Ntotal1=Ninfo1+NCRC1+Ntail1, where Ninfo1 is the number of information bits contained in Part 1 of a HS-SCCH, NCRC1 is the number of CRC bits for Part 1, and Ntail1 is the number of tail bits in Part 1. With 4 HS-SCCHs, the total number of bits carried within a TTI with separate coding on each control channel is M*Ntotal1. For example, assuming that Ninfo1=20, NCRC 1=8 and Ntail 1=8, the total number of bits that must be coded at the base station, or decoded by the UE, for 4 HS-SCCHs equals 144 bits (4*36). This total number of bits is a processing burden, and is an inefficient use of resource due to separate coding that is required for each of the HS-SCCHs. As discussed above, with separate coding, each HS-SCCH carries CRC and tail bits separately.
Likewise, the total number of bits in Part 2 information of a single HS-SCCH is given by Ntotal2=Ninfo2+NCRC2+Ntail2, where Ninfo2 is the number of information bits contained in Part 1, NCRC2 is the number of CRC bits for Part 2, and Ntail2 is the number of tail bits in Part 2 of the signaling message. Since Part 2 is separately coded at the base station and decoded at the UE, the total number of bits to process at the UE is also a processing burden.